Refactor the main nav - Githubissues

Instead of having duplicate HTML elements for the mobile and desktop navs, this PR:

fixes #37
consolidates the HTML,
cleans up the CSS,
adds a semantic <nav> wrapper
removes the dependency on metaquery for the nav
updates the menu script to suit the changes

Mobile screenshots

Before: ![cropped_mobile_before](https://user-images.githubusercontent.com/107569/31007969-e2bf86c6-a55e-11e7-9e1c-264852173404.png) After: ![cropped_mobile_after](https://user-images.githubusercontent.com/107569/31007971-e2bff250-a55e-11e7-8341-a09b578d9ef0.png)

Tablet screenshots

Before: ![cropped_tablet_before](https://user-images.githubusercontent.com/107569/31007972-e2c15b36-a55e-11e7-84bf-160e2f69c5c2.png) After: ![cropped_tablet_after](https://user-images.githubusercontent.com/107569/31007970-e2bff3cc-a55e-11e7-9082-d34e4c3af61a.png)

Desktop screenshots

Before: ![cropped_desktop_before](https://user-images.githubusercontent.com/107569/31007973-e2dcafd0-a55e-11e7-8f02-e52ee649df66.png) After: ![cropped_desktop_after](https://user-images.githubusercontent.com/107569/31007968-e2be5bf2-a55e-11e7-8ea7-80d3e0cd600b.png)

This will cause some conflicts with the #72 PR, but the nav.css changes here supersede the split css nav changes

rubynz / kiwi-ruby-2017

Refactor the main nav #74